Wireless Headphones. Featuring state of the art Bluetooth hardware and software integrated with aptX codec technology, the Koss BT540i is the most advanced Bluetooth headphone designed and engineered by the wireless headphone pioneer, Koss Corporation. Listeners can stay connected using the built-in, dual microphones which work in sync to reduce background noise, producing exceptional call clarity. Easy to locate, on-board controls allow listeners to quickly navigate through songs, play or pause music, volume-up, volume-down, and answer incoming calls. The BT540i Wireless Bluetooth Headphones are outfitted with the all-new precision-tuned Koss PLX40 Dynamic Elements featuring crisp highs, robust mids and smooth lows, perfect for personal listening. Premium leather memory foam cushions close gaps around the ear, creating the perfect seal with an incredibly comfortable fit. The built-in lithium-ion battery delivers more than 12 hours of listening time on a single charge. A detachable audio cable is included and can be used as a bypass should the battery run out. The BT540i folds flat and comes with a travel case for easy portability. Also included is a USB cable for on-the-go charging. Specifications: Frequency range: 10 Hz - 25000 Hz, Impedance: 38 Ohmov, Sensitivty: 100 dB SPL.